1. Field of the Invention
The invention relates generally to shear wave acoustic logging of bore holes and, more particularly, but not by way of limitation, it relates to an improved method and apparatus for obtaining shear wave data related to longer wavelength source frequencies.
2. Description of the Prior Art
The prior art includes numerous types of shear wave logging tool as they have been used for a number of years in obtaining shear wave data from a bore hole in the earths surface. Shear wave generation sources have been utilized both from a position on the earth's surface adjacent the bore hole as well as from the logging tool itself, i.e. the source is an integral part of the tool. In most prior art shear wave applications the generation source has been controlled within higher frequency ranges, e.g. 10-15 kilohertz, and both shear wave source and detectors were positioned in close coupling contact with the bore hole wall in order to provide optimum detection of the shear wave energy. The prior art applications tended toward higher frequency source energies which tended to see the well bore from a microscopic point of view in that a relatively small portion of the well adjacent to the logging tool is seen to vibrate under the influence of those shear wave vibrations whose wavelength is smaller than the bore diameter. Due to this then, depth of penetration or examination by the acoustic waves is limited to the rock structures closely surrounding the bore hole.